The size of a country's population is an important factor in its welfare. The number of children born is the most crucial factor that determines the population of a developed country. Family size decisions, which affect the number of children born, are viewed as consisting of two parts: one, the decision regarding the desired completed family size; and two, the decision regarding the desired spacings between children. Actual family size is the stock that is adjusted in an attempt to achieve the desired completed family size. It is proposed to formulate completely and to estimate empirically a stock- adjustment model of family size. The model that will be utilized will allow for decisions that are altered over time because of a discrepancy between realizations and anticipations due to such factors as unplanned pregnancies, miscarriages, sterility, relative price changes that were not forecasted, and unforeseen financial capital gains and losses. Data from the 1965 National Fertility Study, the 1960 Growth of American Families Study, and the 1955 Growth of American Families Study will be used to estimate parameters by the technique of multiple linear regression. Estimates of desired completed family size and desired spacings between children will be determined simultaneously.